The present invention relates to a fluid level sensor and controller, and more particularly relates to an improved apparatus for measuring, detecting, and controlling liquid levels within a washing machine.
Liquid level sensors for washing machines currently in use comprises two tubes. Each tube having one end connected to the bottom of the washing machine tub and the other end connected to a pressure sensitive switch. The first switch actuates a valve that controls the flow of water going into the washing machine. The second switch when depressed actuates a pump to drain water from the washing machine. As the water level within the washing machine increases, the water within each tube increases, causing the net air pressure over atmospheric pressure to push against the pressure sensitive switch. When the water level in the first tube rises so that the air pressure reaches a set value, the sensing switch toggles and the water flow into the machine is shut off. To drain water from the machine, a pump is turned on. As the water drains, the air pressure in the second tube drops, depressing the second sensing switch, causing a signal to be sent to turn off the pump. By turning the water going into the washing machine and the drain pump off and on, the level within the washing machine can be set.
One problem with this liquid level sensor is that the switch is mechanical and may wear out with use. A mechanical sensor may leak air pressure over time if allowed to remain pressurized. This can cause flooding and subsequent water damage. Another drawback is that the pressure sensitive switch may not have the sensitivity to set the water level with accuracy. Furthermore, the second pressure sensation may not be able to detect when pressure in the second tube is below atmospheric level. Accordingly, the second switch depresses when water is still present in the tub. Consequently, the pump must continue to drain water from the tub for a time period thereafter. Further, this liquid level sensor requires mechanical parts which may be expensive to use and assemble. This liquid level sensor also does not continuously signal the liquid level to a controller but instead indicates only a preset level.
Other methods to sense the liquid level in a tub include a capacitance liquid level sensor. A typical capacitance liquid level sensor comprises a metal rod coated with an insulating material such as Teflon, forming one electrode of a capacitor and a tub wall forming a second electrode of the capacitor. A signal with appropriate RF oscillation is applied across the two electrodes so as to be able to detect and amplify changes in capacitance. These changes provide an output that indicates the liquid level or provides an alarm if the liquid level exceeds a predetermined threshold.
This arrangement suffers from various drawbacks. For example, this arrangement can only be used in tubs which are made of conductive material. Otherwise, additional capacitive elements would have to be provided. This arrangement has further drawbacks in that when used with liquids that contain sticky materials, a build-up on the wires or rods occurs, which results in an inaccurate output indication.
Another liquid level sensor is described in U.S. Pat. No. 4,122,718. A pair of wires are encased in Teflon or an equivalent material. The wires are placed parallel to each other and are spaced equidistantly therein with the Teflon material closed at the terminating end of the liquid level sensor. One drawback of this arrangement is that when the wires are used in a turbulent tub to maintain a constant spacing between the wires, they must be installed within a tube made from a rigid material. A further drawback of this invention is that soap or film may form on the wires and this can possibly affect the capacitance and oscillation frequency of the sensor. A further drawback is that wires do not provide enough surface area to get an accurate capacitance without the addition of more complicated circuitry. This drawback may prevent the indicator from sensing liquid levels down to such small amounts as 1/4 of an inch.
Another drawback of capacitance sensors is that they can be effected by noise and stray capacitance. The capacitor sensor is typically located in the washing machine tub and wired to the capacitance sensing circuitry located a few feet away. The capacitance of the wires changes during washing machine operation as the tub vibrates, resulting in the wires moving around. Accordingly, the sensing circuitry is susceptible to the stray capacitance of the wires between the capacitor sensor and the sensing circuitry. Further, due to the noise generated by the washing machine motor, the output signal of the capacitance circuitry can give erroneous data. Accordingly, due to the noise of the washer motor and turbulence caused by the washer agitator, the accuracy and stability of the sensing apparatus can be affected.